To customize a status page subdomain URL, follow the below steps:

1. Click on the Status page customization icon in the left side menu bar.
2. Under 'General' Section, Edit the 'Custom domain' field.
3. Click on the 'Save and Publish' button to save the changes.

For more information on how to create CNAME record for the Custom domain, Refer this article: Using a Custom domain URL in Freshstatus